1. Field of the Invention
The present invention relates to an optical scanning apparatus and an image forming apparatus using the same. The present invention is suited to an image forming apparatus such as a laser beam printer, a digital copying machine, or a multi-function printer, which adopts an electrophotography process.
2. Description of the Related Art
In the conventional optical scanning apparatus, in order to record image information accurately, it is necessary that field curvature be appropriately corrected over the entire surface to be scanned and that the optical scanning apparatus have a distortion characteristic (fθ-characteristic) with isokinetic property between a field angle (scanning angle) θ and an image height (distance from the center of scanning) Y. Further, it is necessary that a spot diameter on the surface to be scanned be uniform with respect to each image height. There are conventionally proposed various optical scanning apparatuses or imaging optical systems which satisfy such an optical characteristic.
On the other hand, when multiple light beams are deflected for scanning by using a single optical deflector, it is necessary to guide the light beams after the deflection for scanning to individual photosensitive members corresponding to individual colors by separating the light beams in the sub-scanning direction. Therefore, the incident light beams are required to be incident on a deflection plane of the deflecting means perpendicular to the rotational axis of deflecting means from oblique directions (oblique incident optical system in sub-scanning section).
If the light beams are incident on the deflection plane perpendicular to the rotational axis of the deflecting means from an oblique direction, a scanning line is curved on the surface to be scanned. Therefore, a spot deterioration (hereinafter referred to as “spot rotation”) occurs due to a curvature of the scanning line and a deterioration of wavefront aberration.
In particular, a color image forming apparatus, in which the light beams are guided from the optical scanning apparatus to four photosensitive members (photosensitive drums) so as to form latent images and then form Y (yellow), M (magenta), C (cyan) and Bk (black) colors of images of an original on surfaces of the corresponding photosensitive members, has the following problems.
The four color (Y, M, C and Bk) images formed on the individual photosensitive members are superposed on the surfaces of a transfer material such as paper. Therefore, if the scanning lines of the respective optical scanning apparatus corresponding to the photosensitive members have curvatures, the shape of the scanning lines differ among the four colors, which may cause a color drift in the image on the transfer material. As a result, a problem of a conspicuous deterioration in image performance may arise.
In addition, if the spot rotation cannot be appropriately corrected, a design depth becomes significantly small with respect to a case in which the light beam is not incident on the deflection surface obliquely in the sub-scanning section. Therefore, an extreme reduction in manufacturing error is necessary, and hence the products cannot function properly. As means for solving those problems with the oblique incident in the sub-scanning section, some optical scanning apparatuses are variously proposed conventionally.
Japanese Patent Application Laid-Open No. H10-073778 discloses an optical scanning apparatus in which an imaging optical system includes multiple rotation asymmetric lenses, and a meridian line shape connecting sagittal line vertexes of lens surfaces of the rotation asymmetric lenses is curved in the sub-scanning direction (meridian line curving) so as to realize both the curvature of the scanning line and the spot rotation.
In addition, Japanese Patent Application Laid-Open No. 2004-070109 discloses an optical scanning apparatus in which lateral magnification of the imaging optical system in the sub-scanning direction is made constant over the entire field angle, and a surface in which aspherical surface amount in the sagittal line varies is introduced, in which an aspherical surface amount in the sagittal line changes in the main scanning direction, so as to solve the similar problems.
Further, Japanese Patent No. 3453737 and Japanese Patent Application Laid-Open No. 2006-259427 disclose an optical scanning apparatus in which two or more special tilt surfaces (in which the surface normal to the sagittal line changes in the main scanning direction) are introduced in an imaging optical system so as to solve the above-mentioned problems.
If the meridian line curving is to be applied to the optical scanning apparatus disclosed in Japanese Patent Application Laid-Open No. H10-073778, a significant effect tends to be difficult to obtain in an optical surface in which the curvature radius of the sagittal line is reversed, in a vicinity of the position where the sagittal line is reversed.
If the imaging optical system is to be constituted by a single imaging lens in the optical scanning apparatus disclosed in Japanese Patent Application Laid-Open No. 2004-070109, a very thick lens is necessary.
In the optical scanning apparatus disclosed in Japanese Patent No. 3453737 or in Japanese Patent Application Laid-Open No. 2006-259427, the method of changing the tilt angle of the special tilt surface does not necessarily obtain sufficient correction of the curvature of the scanning line or the spot deterioration, and hence there is still a space for improvement. In addition, as described in Japanese Patent No. 3453737 and Japanese Patent Application Laid-Open No. 2006-259427, if the special tilt surface is formed on each lens surface of the two imaging lenses, in other words, if the special tilt surface is formed on different imaging lenses, there may arise a problem that a positional error between the two imaging lenses causes a surface position shift between the special tilt surfaces in the main scanning direction and in the sub-scanning direction.